1. Technical Field of the Invention
The present invention relates to the preparation of hydrophilic polymers and copolymers in powder form, the principal characteristic of which is, if they are not crosslinked, the high speed at which they dissolve in water, and which are well suited, inter alia, as flocculants for water treatments, as sludge- dehydrating agents, as thickeners, or else as retention agents in the papermaking industry and, if they are partially crosslinked, their high absorption of water and of aqueous fluids, which renders them well suited as superabsorbents, both for the production of articles of hygiene and for cable manufacture.
The hydrophilic polymers and copolymers according to the present invention are the linear or branched, uncrosslinked or partially crosslinked (co)polymers having a molecular weight of several million daltons, which are prepared via polymerization in a disperse medium, in the first stage of any which process the continuous phase is an organic phase and the disperse phase an aqueous solution of the constituent (co)monomers.
2. Description of the Prior Art
The aforesaid so-called inverse suspension polymerization techniques have long been known to this art and are described in very many patents. Compare, for example, U.S. Pat. Nos. 3,957,739 and 4,093,776 for a polymerization process in one stage, or EP-441,507 (Sumitomo Seika Chem. Co.) for a process entailing two or more stages of successive introduction of the monomer to be polymerized.
An important parameter which is common to all these techniques is the use of at least one surfactant whose hydro-lipophilic balance (HLB) ranges from 3 to 6 and whose function is to stabilize the inverse suspension.
At termination of these inverse suspension polymerization processes, the ultimate polymer is in the form of a powder of spherical morphology and having a particle size distribution ranging from 50 to 400 .mu.m, and its dissolving or swelling in water is slow, and this considerably reduces its merit, especially for applications such as the thickening of aqueous solutions or water treatment.
In other inverse suspension polymerization processes, surfactants of HLB of from 8 to 12 have been used, for example those described in EP-36,463 (Seitetsu Kagaku Co) or FR 2,251,573 (Nobel Hoechst Chimie). The inverse suspension is produced therein at low temperature in the same manner as in the preceding techniques, namely, to begin, a dispersion of the aqueous solution of monomers in the form of droplets is established in the organic continuous medium and the temperature is increased to initiate the polymerization. Here, however, an inversion of the phases is observed during the polymerization: a continuous gel forms, which the mechanical effect of the stirring converts into granules whose particle size ranges from 10 .mu.m to 600 .mu.m. This phase inversion mechanism has been explained in "Inverse Suspension Polymerization of Acrylamide" in European Polymer Journal, vol. 18, pages 639 to 645 (1982). The powder grains thus obtained have a very specific, very distorted or irregular morphology which resembles that of "raspberries" or of "cauliflowers", and no longer that of spherical beads. Their macroporosity is high and consequently their specific surface is on average 3 to 10 times greater than that of a powder of spherical grains of equivalent particle size distribution. This large specific surface plays a very important role in the speed at which the powder dissolves (or swells) in water and aqueous fluids. The kinetics of dissolution of such grains are rapid and, as a result, these powders are of great interest for the abovementioned applications. However, the great disadvantage presented by these inverse suspension processes with phase inversion is their output efficiency, which is limited as a result of the overloading of the reactor with the organic phase. It has been determined, indeed, that the use of surfactants of HLB of from 8 to 12 does not permit operating at a volumetric ratio of aqueous phase to organic phase (which will be referred to as an A/O ratio hereinafter) which is high. According to the prior art, this ratio is very critical and this is confirmed in industrial practice: it is essential to adopt an A/O ratio lower than 0.6 and preferably lower than 0.5 in order that the initial inverse suspension before polymerization should be stabilized. At A/O ratios higher than 0.6 the surfactant of HLB of from 8 to 12 is no longer capable of ensuring an equilibrium between the coalescence and the rupture of the droplets in suspension and the system sets solid during polymerization.